Although Apple doesn't call this out obviously, they typically support 3 years of OS's with security updates, so a mac that can only run Sierra today would still get a security update, but otherwise they do not receive bug fixes or any other type of update after the new OS is released. So after Catalina is released, High Sierra will still be getting security updates for another year, but Sierra is now out in the cold. So basically you still have ~2 years of secure usage once the first version that doesn't run on your machine is released. As much as I prefer the 2015 design to the newer ones, it's a hard sell now because it is 4 years old already and that much closer to the chopping edge of support. But for the right price it would be a good deal. Unfortunately, you have to make that call yourself and decide if you think it's worth it. I'm not sure what your Mac guy meant by no one being able to update new OS's anymore after Mojave, that statement doesn't make sense. Most new versions of the OS have lost support for some older machines each year, and I don't see that trend stopping or changing in the foreseeable future even though I would much prefer to go back to 2-3 year long release cycles on MacOS.